index: don't store commit ids in sorted lookup table to save disk space #3087
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This reduces the index file size. In my linux mirror repo containing 1591524
commits, the initial index file shrank from 122MB to 92MB. In theory, this
makes commit id lookup slow because of additional indirection and cache miss,
but I don't see significant difference. In mid-size repo, this is actually a
bit faster thanks to smaller index reads.
Alternatively, the commit id field could be removed from the CommitGraphEntry,
but doing that would introduce indirect lookup there, and the index disk size
isn't as small as this change.
- jj-0 baseline 122MB
- jj-1 shrink CommitLookupEntry (this) 92MB
- jj-3 shrink CommitGraphEntry 98MB
Mid-size repo, "log" with default template
```
% hyperfine --sort command --warmup 3 --runs 20 -L bin jj-0,jj-1,jj-2,jj-3 \
-s "target/release-with-debug/{bin} -R ~/mirrors/linux debug reindex" \
"target/release-with-debug/{bin} -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=\"\"'"
Benchmark 1: target/release-with-debug/jj-0 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 177.7 ms ± 12.9 ms [User: 96.3 ms, System: 81.5 ms]
Range (min … max): 156.8 ms … 191.2 ms 20 runs
Benchmark 2: target/release-with-debug/jj-1 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 169.8 ms ± 13.8 ms [User: 93.3 ms, System: 76.6 ms]
Range (min … max): 151.1 ms … 191.5 ms 20 runs
Benchmark 4: target/release-with-debug/jj-3 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 170.3 ms ± 13.4 ms [User: 90.1 ms, System: 79.7 ms]
Range (min … max): 154.8 ms … 186.2 ms 20 runs
Relative speed comparison
1.05 ± 0.11 target/release-with-debug/jj-0 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
1.00 target/release-with-debug/jj-1 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
1.00 ± 0.11 target/release-with-debug/jj-3 -R ~/mirrors/linux --ignore-working-copy log -r.. -l100 --config-toml='revsets.short-prefixes=""'
```
Small repo, "log" thousands of commits with -T"commit_id.shortest()"
```
% hyperfine --sort command --warmup 3 --runs 100 -L bin jj-0,jj-1,jj-2,jj-3 \
-s "target/release-with-debug/{bin} -R ~/mirrors/git debug reindex" \
"target/release-with-debug/{bin} -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=\"\"'"
Benchmark 1: target/release-with-debug/jj-0 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 179.3 ms ± 12.8 ms [User: 149.7 ms, System: 29.6 ms]
Range (min … max): 155.2 ms … 191.0 ms 100 runs
Benchmark 2: target/release-with-debug/jj-1 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 179.1 ms ± 13.7 ms [User: 148.5 ms, System: 30.5 ms]
Range (min … max): 157.2 ms … 196.7 ms 100 runs
Benchmark 4: target/release-with-debug/jj-3 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
Time (mean ± σ): 178.2 ms ± 13.6 ms [User: 148.7 ms, System: 29.6 ms]
Range (min … max): 156.5 ms … 191.7 ms 100 runs
Relative speed comparison
1.01 ± 0.11 target/release-with-debug/jj-0 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
1.01 ± 0.11 target/release-with-debug/jj-1 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
1.01 ± 0.11 target/release-with-debug/jj-3 -R ~/mirrors/git --ignore-working-copy log -r.. -l5000 -T'commit_id.shortest()' --config-toml='revsets.short-prefixes=""'
```
martinvonz
approved these changes
Feb 18, 2024
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CHANGELOG.md